Two CMOS oscillators. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Two CMOS oscillators. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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In experiments with the widely used K176LA7 CMOS chip, the author managed to implement two simple generators, which we offer to readers.

In amateur radio practice, there is often a need for a highly stable generator, but a quartz resonator with the desired operating frequency cannot be found. If there is a resonator with a higher frequency, then you can, for example, make an oscillator with quartz frequency stabilization, and then use a divider to lower it to the desired value. Such a device usually requires at least two microcircuits. Meanwhile, when a radio amateur has a resonator with an operating frequency three times higher than required, the problem can be solved much easier. In the generator, the circuit of which is shown in Fig. 1, the author used a quartz resonator at a frequency of 500 kHz, and the square wave at the output of the generator had a frequency of 166.(6) kHz. You can take resonators for other frequencies (from tens of kHz to several MHz), but you will have to experimentally select capacitor C1 and resistor R1. (The higher the frequency, the lower the ratings, and vice versa).

But how does such a generator work if quartz has no resonances at frequencies below the fundamental one? The point is that in Fig. 1 RC generator has all the conditions for self-excitation. Indeed, the parallel capacitance of the quartz and the quartz holder forms a positive feedback circuit, and the resistor R1 closes the DC feedback circuit, which ensures the linear operation of the first two elements of the DD1 microcircuit. By selecting resistor R1 and capacitor C1, the oscillator frequency is set slightly lower than the operating frequency of the quartz resonator, divided by three. Steep fronts of rectangular pulses excite the resonator at the th fundamental frequency. The voltage arising at its terminals with a frequency of 500 kHz synchronizes the RC generator, and very hard, accurate to the phase.

Two CMOS oscillators

All this can be observed using an oscilloscope by connecting a probe with a small input capacitance (so as not to disrupt the operation of the generator) to the right output of the quartz resonator according to the circuit. The screen shows how smaller amplitude sinusoidal oscillations with a frequency of 166 kHz are superimposed on rectangular oscillations with a frequency of 6. (500) kHz. The synchronization bandwidth of the described generator is quite large, so such destabilizing factors as changes in certain limits of the supply voltage, temperature and element ratings have practically no effect on its operation. The stability of its frequency is entirely determined by the used quartz resonator.

Another generator, in contrast to the one just described, has a very wide tuning range, and here it is no longer necessary to talk about frequency stability - it is completely (temperature dependence has not been studied) is determined by the stability of the control voltage. The generator circuit is shown in fig. 2. It has only one blocking capacitor, which prevents the generator oscillations from entering the frequency control circuit and protects it from external interference. It does not participate in the operation of the generator itself. All elements of the microcircuit are connected in series, the generator is assembled on the first three of them, and the output buffer stage is on the fourth.

Two CMOS oscillators

The feedback circuit is formed by the resistor R1, it is negative for direct current and therefore provides a linear mode of operation of the generator elements. In each of them, the signal is delayed for a certain time, and the duration of this delay strongly depends on the supply voltage - the higher it is, the less the delay. The oscillation phase shift is proportional to the product of the delay time and the frequency. At a sufficiently high frequency, the phase shift in each element of the microcircuit reaches 60, and in all three - 180 °. As a result, the OOS turns into a positive one and the generator is excited at this frequency. When the supply voltage is increased from 3 to 12 V, the generator frequency changes from about 300 kHz to 6 MHz, i.e., 20 times. The current consumption increases in this case from fractions of a milliamp to 2 mA. In order for the generator to cover, for example, the medium wave range (500 ... 1600 kHz), the supply voltage must change from only 3,5 to 5 V. The frequency range can be changed by selecting resistor R1.

The advantage of the described generator is its exceptional simplicity, and the main drawback is the strong dependence of the output voltage on frequency.

Author: V.Polyakov, Moscow

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